Inhibitors of amyloid β-protein aggregation mediated by GM1-containing raft-like membranes

Mechanisms Linking Obesity, Insulin Resistance, and Alzheimer's Disease: Effects of Polyphenols and Omega-3 Polyunsaturated Fatty Acids

Alzheimer's disease (AD) is a complex neurodegenerative disorder characterized by progressive cognitive decline, memory loss, and behavioral changes. It poses a significant global health challenge. AD is associated with the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles (NFTs) in the brain, along with chronic inflammation, dysfunctional neurons, and synapse loss. While the prevalence of AD continues to rise, the current FDA-approved drugs offer only limited effectiveness. Emerging evidence suggests that obesity, insulin resistance (IR), and type 2 diabetes mellitus (T2DM) are also implicated in AD pathogenesis, with epidemiological studies and animal models confirming the impact of IR on Aβ accumulation, and high-fat diets also exacerbating Aβ accumulation. Since neuroinflammation activated by Aβ involves the nuclear factor kappa-light-chain-enhancer of the activated B cell (NF-κB) pathway, the inhibition of NF-κB and NLRP3 inflammasome activation are potential therapeutic strategies in AD. Bioactive compounds, including polyphenols (resveratrol, epigallocatechin-3-gallate, curcumin, and quercetin), and omega-3 polyunsaturated fatty acids, show promising results in animal studies and clinical trials for reducing Aβ levels, improving cognition and modulating the signaling pathways implicated in AD. This review explores the interplay between obesity, IR, inflammation, and AD pathology, emphasizing the potential of dietary compounds and their role in reducing inflammation, oxidative stress, and cognitive decline, as viable strategies for AD prevention and treatment. By integrating epidemiological findings, observational studies, and clinical trials, this review aims to provide a comprehensive understating of how metabolic dysfunctions and bioactive compounds influence AD progression.

Discovery of novel coumarin triazolyl and phenoxyphenyl triazolyl derivatives targeting amyloid beta aggregation-mediated oxidative stress and neuroinflammation for enhanced neuroprotection

This study involved designing, synthesizing, and evaluating the protective potential of compounds on microglial cells (BV-2 cells) and neurons (SH-SY5Y cells) against cell death induced by Aβ1-42. It aimed to identify biologically specific activities associated with anti-Aβ aggregation and understand their role in oxidative stress initiation and modulation of proinflammatory cytokine expression. Actively designed compounds CE5, CA5, PE5, and PA5 showed protective effects on BV-2 and SH-SY5Y cells, with cell viability ranging from 60.78 ± 2.32% to 75.38 ± 2.75% for BV-2 cells and 87.21% ± 1.76% to 91.55% ± 1.78% for SH-SY5Y cells. The transformation from ester in CE5 to amide in CA5 resulted in significant antioxidant properties. Molecular docking studies revealed strong binding of CE5 to critical Aβ aggregation regions, disrupting both intra- and intermolecular formations. TEM assessment supported CE5's anti-Aβ aggregation efficacy. Structural variations in PE5 and PA5 had diverse effects on IL-1β and IL-6, suggesting further specificity studies for Alzheimer's disease. Log P values suggested potential blood-brain barrier permeation for CE5 and CA5, indicating suitability for CNS drug development. In silico ADMET and toxicological screening revealed that CE5, PA5, and PE5 have favorable safety profiles, while CA5 shows a propensity for hepatotoxicity. According to this prediction, coumarin triazolyl derivatives are likely to exhibit mutagenicity. Nevertheless, CE5 and CA5 emerge as promising lead compounds for Alzheimer's therapeutic intervention, with further insights expected from subsequent in vivo studies.

Effects of Ferulic Acid on Cognitive Function: A Systematic Review

SCOPE

Plant (poly) phenolic compounds have been reported to decrease the risk of developing dementia and have been associated with maintenance of cognitive performance in normal ageing. Ferulic acid (FA) is a phenolic acid, present in a wide variety of foods including cereals, fruits, vegetables, and coffee. The aim of this systematic review is to examine the effect of FA on cognitive function in humans and animals.

METHODS AND RESULTS

The search terms "Ferulic acid AND cognit*" and "Ferulic acid OR feruloyl OR ferula AND (memory OR attention OR learning OR recognition)" are used in Web of Science, Scopus, PubMED, OVID (Medline/PsycInfo), and CINAHL through October 2023. No human studies are identified that matched the inclusion criteria. Twenty-six animal studies are identified. A small number (n = 5) of these studies examined FA in healthy animals whilst the remainder examined animal models of dementia. Alzheimer's disease (n = 11) is the most prevalent model.

CONCLUSION

Overall, results from studies employing disease models suggest that FA ameliorates induced cognitive decline in a time and dose-dependent manner. Similarly, studies in healthy animals show a beneficial effect of FA. However, further studies are required to determine the effects of FA on human cognitive function.

Exploring the Therapeutic Potential of Phytochemicals in Alzheimer’s Disease: Focus on Polyphenols and Monoterpenes

Alzheimer’s disease (AD) is a chronic, complex neurodegenerative disorder mainly characterized by the irreversible loss of memory and cognitive functions. Different hypotheses have been proposed thus far to explain the etiology of this devastating disorder, including those centered on the Amyloid-β (Aβ) peptide aggregation, Tau hyperphosphorylation, neuroinflammation and oxidative stress. Nonetheless, the therapeutic strategies conceived thus far to treat AD neurodegeneration have proven unsuccessful, probably due to the use of single-target drugs unable to arrest the progressive deterioration of brain functions. For this reason, the theoretical description of the AD etiology has recently switched from over-emphasizing a single deleterious process to considering AD neurodegeneration as the result of different pathogenic mechanisms and their interplay. Moreover, much relevance has recently been conferred to several comorbidities inducing insulin resistance and brain energy hypometabolism, including diabetes and obesity. As consequence, much interest is currently accorded in AD treatment to a multi-target approach interfering with different pathways at the same time, and to life-style interventions aimed at preventing the modifiable risk-factors strictly associated with aging. In this context, phytochemical compounds are emerging as an enormous source to draw on in the search for multi-target agents completing or assisting the traditional pharmacological medicine. Intriguingly, many plant-derived compounds have proven their efficacy in counteracting several pathogenic processes such as the Aβ aggregation, neuroinflammation, oxidative stress and insulin resistance. Many strategies have also been conceived to overcome the limitations of some promising phytochemicals related to their poor pharmacokinetic profiles, including nanotechnology and synthetic routes. Considering the emerging therapeutic potential of natural medicine, the aim of the present review is therefore to highlight the most promising phytochemical compounds belonging to two major classes, polyphenols and monoterpenes, and to report the main findings about their mechanisms of action relating to the AD pathogenesis.

Elucidation of Complex Dynamic Intermolecular Interactions in Membranes

Biomembranes composed of various proteins and lipids play important roles in cellular functions, such as signal transduction and substance transport. In addition, some bioactive peptides and pathogenic proteins target membrane proteins and lipids to exert their effects. Therefore, an understanding of dynamic and complex intermolecular interactions among these membrane constituents is needed to elucidate their mechanisms. This review summarizes the major research carried out in the author's laboratory on how lipids and their inhomogeneous distributions regulate the structures and functions of antimicrobial peptides and Alzheimer's amyloid β-protein. Also, how to detect transmembrane helix-helix and membrane protein-protein interactions and how they are modulated by lipids are discussed.

Vitamin B12 Inhibits Aβ Fibrillation and Disaggregates Preformed Fibrils in the Presence of Synthetic Neuronal Membranes

The aggregation of amyloid β (Aβ) peptide with subsequent formation of fibrils which deposit in senile plaques is considered one of the key triggers of Alzheimer's disease (AD). Molecules targeting the inhibition of Aβ fibrillation and/or the disruption of Aβ fibrils are thus promising approaches for the medical prevention and treatment of AD. However, amyloid formation is a complex process strongly influenced by the cellular environment, such as cell membranes, which may affect the effectiveness of therapeutic molecules. In this study, the effect of the vitamin B12 (VB12) on the formation and disaggregation of Aβ_1-42 fibrils was investigated in the presence of artificial neuronal membranes mimicked by liposomes. Evidence showed that VB12 slows down the Aβ fibrillization and reduces the content of fibrils in aqueous solution. Moreover, the vitamin exhibited a strong ability to disrupt preformed fibrils. However, the presence of lipid vesicles compromised the VB12's antiamyloidogenic properties due to the competitive interaction of the vitamin with the lipid membrane and the Aβ peptide. Even so, VB12 was effective in inhibiting the fibril formation and disaggregating fibrils in the lipid membrane environment. Thereby, these results indicate that VB12 could be a promising molecule both for the prevention and cure of AD, thus warranting its study in animal models.

Therapeutic Strategies Targeting Amyloid-β in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder linked to protein misfolding and aggregation. AD is pathologically characterized by senile plaques formed by extracellular Amyloid-β (Aβ) peptide and Intracellular Neurofibrillary Tangles (NFT) formed by hyperphosphorylated tau protein. Extensive synaptic loss and neuronal degeneration are responsible for memory impairment, cognitive decline and behavioral dysfunctions typical of AD. Amyloidosis has been implicated in the depression of acetylcholine synthesis and release, overactivation of N-methyl-D-aspartate (NMDA) receptors and increased intracellular calcium levels that result in excitotoxic neuronal degeneration. Current drugs used in AD treatment are either cholinesterase inhibitors or NMDA receptor antagonists; however, they provide only symptomatic relief and do not alter the progression of the disease. Aβ is the product of Amyloid Precursor Protein (APP) processing after successive cleavage by β- and γ-secretases while APP proteolysis by α-secretase results in non-amyloidogenic products. According to the amyloid cascade hypothesis, Aβ dyshomeostasis results in the accumulation and aggregation of Aβ into soluble oligomers and insoluble fibrils. The former are synaptotoxic and can induce tau hyperphosphorylation while the latter deposit in senile plaques and elicit proinflammatory responses, contributing to oxidative stress, neuronal degeneration and neuroinflammation. Aβ-protein-targeted therapeutic strategies are thus a promising disease-modifying approach for the treatment and prevention of AD. This review summarizes recent findings on Aβ-protein targeted AD drugs, including β-secretase inhibitors, γ-secretase inhibitors and modulators, α-secretase activators, direct inhibitors of Aβ aggregation and immunotherapy targeting Aβ, focusing mainly on those currently under clinical trials.

Date palm (Phoenix dactylifera L.) fruit's polyphenols as potential inhibitors for human amylin fibril formation and toxicity in type 2 diabetes

BACKGROUND

β-Cell death is the key feature of type 2 diabetes mellitus (T2DM). The misfolding of human Islet Amyloid Polypeptide (hIAPP) is regarded as one of the causative factors of T2DM. Recent studies suggested that a diet based on date fruits presents various health benefits, as these fruits are naturally enriched in plant polyphenols.

METHOD

In this study, we used a broad biophysical approach, using cell biology techniques and bioinformatic tools, to demonstrate that various polyphenols from date palm (Phoenix dactylifera L.) fruit significantly inhibited hIAPP aggregation and cytotoxicity.

RESULT

Our results suggest that all of the polyphenols showed inhibitory effects, albeit varied, on the formation of toxic hIAPP amyloids. Correlation between cell viability assay, permeabilization of synthetic phospholipid vesicles tests, and ANS florescence measurements, revealed that both classes of polyphenols protected INS-1E cells from the toxicity of amylin aggregates. Docking results showed that the used polyphenols physically interacted with both hIAPP amyloidogenic region (residues Ser20-Ser29) and the non-amyloidogenic regions via hydrophobic and hydrogen interactions, thus reducing aggregation levels.

CONCLUSION

These findings highlight the benefits of consuming dates and the great potential of its polyphenols as a potential therapy for the prevention and treatment of T2DM as well as for many other amyloid-related diseases.

Aβ-ganglioside interactions in the pathogenesis of Alzheimer's disease

It is widely accepted that the abnormal self-association of amyloid β-protein (Aβ) is central to the pathogenesis of Alzheimer's disease, the most common form of dementia. Accumulating evidence, both in vivo and in vitro, suggests that the binding of Aβ to gangliosides, especially monosialoganglioside GM1, plays an important role in the aggregation of Aβ. This review summarizes the molecular details of the binding of Aβ to ganglioside-containing membranes and subsequent structural changes, as revealed by liposomal and cellular studies. Furthermore, mechanisms of cytotoxicity by aggregated Aβ are also discussed.

The Additive Effects of Low Dose Intake of Ferulic Acid, Phosphatidylserine and Curcumin, Not Alone, Improve Cognitive Function in APPswe/PS1dE9 Transgenic Mice

Alzheimer's disease (AD) is the most common form of dementia and its prevention and treatment is a worldwide issue. Many natural components considered to be effective against AD have been identified. However, almost all clinical trials of these components for AD reported inconclusive results. We thought that multiple factors such as amyloid β (Aβ) and tau progressed the pathology of AD and that a therapeutic effect would be obtained by using multiple active ingredients with different effects. Thus, in this study, we treated ferulic acid (FA), phosphatidylserine (PS) and curcumin (Cur) in combination or alone to APPswe/PS1dE9 transgenic mice and evaluated cognitive function by Y-maze test. Consequently, only the three-ingredient group exhibited a significant improvement in cognitive function compared to the control group. In addition, we determined the amounts of Aβ, brain-derived neurotrophic factor (BDNF), interleukin (IL)-1β, acetylcholine and phosphorylated tau in the mouse brains after the treatment. In the two-ingredient (FA and PS) group, a significant decrease in IL-1β and an increasing trend in acetylcholine were observed. In the Cur group, significant decreases in Aβ and phosphorylated tau and an increasing trend in BDNF were observed. In the three-ingredient group, all of them were observed. These results indicate that the intake of multiple active ingredients with different mechanisms of action for the prevention and treatment of AD.

Kinetic study of Aβ(1-42) amyloidosis in the presence of ganglioside-containing vesicles

Alzheimer's disease (AD) is characterized by the amyloid-beta peptide (Aβ) misfolding to form aberrant amyloid aggregates in the brain. Although recent evidence implicates that amyloid deposition in vivo is highly related to biomembranes, how the characteristic lipid components of neuronal membranes mediate this process remains to be fully elucidated. Herein, we established vesicle models to mimic exosomes and investigated their influence on the kinetics of Aβ(1-42) amyloidosis. By using ternary vesicles composed of three brain lipids monosialoganglioside GM1, cholesterol and sphingomyelin, we found that GM1 could regulate peptide fibrillation by facilitating the conformational transition of Aβ(1-42), and further quantitatively analyzed the influence of GM1-containing vesicles on the kinetics of Aβ(1-42) fibrillation. In addition, GM1-containing vesicles induced the formation of Aβ(1-42) fibrils at low concentrations, and these fibrils were toxic to PC12 cells. By analyzing the role of GM1 in this ternary mixture of membranes at the molecular level, we confirmed that GM1 clusters are presented as attachment sites for peptides, thus promoting the fibrillation of Aβ(1-42).

Inhibition of Amyloid Fibrillation by Small Molecules and Nanomaterials: Strategic Development of Pharmaceuticals Against Amyloidosis

Amyloid fibrils are a special class of self-assembled protein molecules, which exhibit various toxic effects in cells. Different physiological disorders such as Alzheimer's, Parkinson's, Huntington's diseases, etc. happen due to amyloid formation and lack of proper cellular mechanism for the removal of fibrils. Therefore, inhibition of amyloid fibrillation will find immense applications to combat the diseases associated with amyloidosis. The development of therapeutics against amyloidosis is definitely challenging and numerous strategies have been followed to find out anti-amyloidogenic molecules. Inhibition of amyloid aggregation of proteins can be achieved either by stabilizing the native conformation or by decreasing the chances of assembly formation by the unfolded/misfolded structures. Various small molecules such as naturally occurring polyphenols, flavonoids, small organic molecules, surfactants, dyes, chaperones, etc. have demonstrated their capability to interrupt the amyloid fibrillation of proteins. In addition to that, in last few years, different nanomaterials were evolved as effective therapeutic inhibitors against amyloidosis. Aromatic and hydrophobic interactions between the partially unfolded protein molecules and the inhibitors had been pointed as a general mechanism for inhibition. In this review article, we are presenting an overview on the inhibition of amyloidosis by using different small molecules (both natural and synthetic origin) as well as nanomaterials for development of pharmaceutical strategies against amyloid diseases.

Inhibition of protein misfolding and aggregation by natural phenolic compounds

Protein misfolding and aggregation into fibrillar deposits is a common feature of a large group of degenerative diseases affecting the central nervous system or peripheral organs, termed protein misfolding disorders (PMDs). Despite their established toxic nature, clinical trials aiming to reduce misfolded aggregates have been unsuccessful in treating or curing PMDs. An interesting possibility for disease intervention is the regular intake of natural food or herbal extracts, which contain active molecules that inhibit aggregation or induce the disassembly of misfolded aggregates. Among natural compounds, phenolic molecules are of particular interest, since most have dual activity as amyloid aggregation inhibitors and antioxidants. In this article, we review many phenolic natural compounds which have been reported in diverse model systems to have the potential to delay or prevent the development of various PMDs, including Alzheimer's and Parkinson's diseases, prion diseases, amyotrophic lateral sclerosis, systemic amyloidosis, and type 2 diabetes. The lower toxicity of natural compounds compared to synthetic chemical molecules suggest that they could serve as a good starting point to discover protein misfolding inhibitors that might be useful for the treatment of various incurable diseases.

Lipid-Modified Graphene-Transistor Biosensor for Monitoring Amyloid-β Aggregation

A graphene field-effect transistor (G-FET) with the spacious planar graphene surface can provide a large-area interface with cell membranes to serve as a platform for the study of cell membrane-related protein interactions. In this study, a G-FET device paved with a supported lipid bilayer (referred to as SLB/G-FET) was first used to monitor the catalytic hydrolysis of the SLB by phospholipase D. With excellent detection sensitivity, this G-FET was also modified with a ganglioside G_M1-enriched SLB (G_M1-SLB/G-FET) to detect cholera toxin B. Finally, the GM1-SLB/G-FET was employed to monitor amyloid-beta 40 (Aβ40) aggregation. In the early nucleation stage of Aβ40 aggregation, while no fluorescence was detectable with traditional thioflavin T (ThT) assay, the prominent electrical signals probed by G_M1-SLB/G-FET demonstrate that the G-FET detection is more sensitive than the ThT assay. The comprehensive kinetic information during the Aβ40 aggregation could be collected with a G_M1-SLB/G-FET, especially covering the kinetics involved in the early stage of Aβ40 aggregation. These experimental results suggest that SLB/G-FETs hold great potential as a powerful biomimetic sensor for versatile investigations of membrane-related protein functions and interaction kinetics.

Expression and function of Abcg4 in the mouse blood-brain barrier: role in restricting the brain entry of amyloid-β peptide

ABCG4 is an ATP-binding cassette transmembrane protein which has been shown, in vitro, to participate in the cellular efflux of desmosterol and amyloid-β peptide (Aβ). ABCG4 is highly expressed in the brain, but its localization and function at the blood-brain barrier (BBB) level remain unknown. We demonstrate by qRT-PCR and confocal imaging that mouse Abcg4 is expressed in the brain capillary endothelial cells. Modelling studies of the Abcg4 dimer suggested that desmosterol showed thermodynamically favorable binding at the putative sterol-binding site, and this was greater than for cholesterol. Additionally, unbiased docking also showed Aβ binding at this site. Using a novel Abcg4-deficient mouse model, we show that Abcg4 was able to export Aβ and desmosterol at the BBB level and these processes could be inhibited by probucol and L-thyroxine. Our assay also showed that desmosterol antagonized the export of Aβ, presumably as both bind at the sterol-binding site on Abcg4. We show for the first time that Abcg4 may function in vivo to export Aβ at the BBB, in a process that can be antagonized by its putative natural ligand, desmosterol (and possibly cholesterol).

Neuritogenic activity of bi-functional bis-tryptoline triazole

Alzheimer's disease (AD) is a common neurodegenerative disorder, one of the hallmarks of which is the deposition of aggregated β-amyloid peptides (Aβ40,42) as plaques in the brain. Oligomers of these peptides have been reported to be toxic and to inhibit neurite outgrowth, as evidenced by neurite dystrophy and significant loss of synaptic connectivity of neurons in the AD brain resulting in cognitive decline. These peptides also react with biological metal in the brain to generate free radicals, thereby aggravating neuronal cell injury and death. Herein, multifunctional triazole-based compounds acting on multiple targets, namely β-secretase (BACE1), β-amyloid peptides (Aβ) as well as those possessing metal chelation and antioxidant properties, were developed and evaluated for neuritogenic activity in P19-derived neurons. At the non-cytotoxic concentration (1nM), all multifunctional compounds significantly enhanced neurite outgrowth. New bis-tryptoline triazole (BTT) increased the neurite length and neurite number, by 93.25% and 136.09% over the control, respectively. This finding demonstrates the ability of multifunctional compounds targeting Aβ to enhance neurite outgrowth in addition to their neuroprotective action.

A Comprehensive Spectroscopic and Computational Investigation to Probe the Interaction of Antineoplastic Drug Nordihydroguaiaretic Acid with Serum Albumins

Exogenous drugs that are used as antidote against chemotheray, inflammation or viral infection, gets absorbed and interacts reversibly to the major serum transport protein i.e. albumins, upon entering the circulatory system. To have a structural guideline in the rational drug designing and in the synthesis of drugs with greater efficacy, the binding mechanism of an antineoplastic and anti-inflammatory drug Nordihydroguaiaretic acid (NDGA) with human and bovine serum albumins (HSA & BSA) were examined by spectroscopic and computational methods. NDGA binds to site II of HSA with binding constant (Kb) ~105 M-1 and free energy (ΔG) ~ -7.5 kcal.mol-1. It also binds at site II of BSA but with lesser binding affinity (Kb) ~105 M-1 and ΔG ~ -6.5 kcal.mol-1. The negative value of ΔG, ΔH and ΔS for both the albumins at three different temperatures confirmed that the complex formation process between albumins and NDGA is spontaneous and exothermic. Furthermore, hydrogen bonds and hydrophobic interactions are the main forces involved in complex formation of NDGA with both the albumins as evaluated from fluorescence and molecular docking results. Binding of NDGA to both the albumins alter the conformation and causes minor change in the secondary structure of proteins as indicated by the CD spectra.

How Do the Size, Charge and Shape of Nanoparticles Affect Amyloid β Aggregation on Brain Lipid Bilayer?

Here, we studied the effect of the size, shape, and surface charge of Au nanoparticles (AuNPs) on amyloid beta (Aβ) aggregation on a total brain lipid-based supported lipid bilayer (brain SLB), a fluid platform that facilitates Aβ-AuNP aggregation process. We found that larger AuNPs induce large and amorphous aggregates on the brain SLB, whereas smaller AuNPs induce protofibrillar Aβ structures. Positively charged AuNPs were more strongly attracted to Aβ than negatively charged AuNPs, and the stronger interactions between AuNPs and Aβ resulted in fewer β-sheets and more random coil structures. We also compared spherical AuNPs, gold nanorods (AuNRs), and gold nanocubes (AuNCs) to study the effect of nanoparticle shape on Aβ aggregation on the brain SLB. Aβ was preferentially bound to the long axis of AuNRs and fewer fibrils were formed whereas all the facets of AuNCs interacted with Aβ to produce the fibril networks. Finally, it was revealed that different nanostructures induce different cytotoxicity on neuroblastoma cells, and, overall, smaller Aβ aggregates induce higher cytotoxicity. The results offer insight into the roles of NPs and brain SLB in Aβ aggregation on the cell membrane and can facilitate the understanding of Aβ-nanostructure co-aggregation mechanism and tuning Aβ aggregate structures.

Europium as an inhibitor of Amyloid-β(1-42) induced membrane permeation

•

Europium ions complex with GM1 gangliosides in phospholipid membranes.

•

Europium ions cause inhibition Aβ–membrane interactions.

•

Europium blocks an Aβ receptor protecting against membrane permeation.

•

Discrete Aβ binding events correlate to specific membrane permeation events.

Soluble Amyloid-beta (Aβ) oligomers are a source of cytotoxicity in Alzheimer’s disease (AD). The toxicity of Aβ oligomers may arise from their ability to interact with and disrupt cellular membranes mediated by GM1 ganglioside receptors within these membranes. Therefore, inhibition of Aβ–membrane interactions could provide a means of preventing the toxicity associated with Aβ. Here, using Surface Plasmon field-enhanced Fluorescence Spectroscopy, we determine that the lanthanide, Europium III chloride (Eu³⁺), strongly binds to GM1 ganglioside-containing membranes and prevents the interaction with Aβ42 leading to a loss of the peptides ability to cause membrane permeation. Here we discuss the molecular mechanism by which Eu³⁺ inhibits Aβ42-membrane interactions and this may lead to protection of membrane integrity against Aβ42 induced toxicity.

How do membranes initiate Alzheimer's Disease? Formation of toxic amyloid fibrils by the amyloid β-protein on ganglioside clusters

Alzheimer's disease (AD), a severe neurodegenerative disorder, causes more than half of dementia cases. According to the popular "Aβ hypothesis" to explain the mechanism of this disease, amyloid β-peptides (Aβ) of 39-43 amino acid residues aggregate and deposit onto neurons, igniting the neurotoxic cascade of the disease. Therefore, researchers studying AD would like to elucidate the mechanisms by which essentially water-soluble but hydrophobic Aβ aggregates under pathological conditions. Most researchers have investigated the aggregation of Aβ in aqueous solution, and they concluded that the final aggregation product, the amyloid fibrils, were less toxic than the component peptide oligomers. They consequently shifted their interests to more toxic "soluble oligomers", structures that form as intermediates or off-pathway products during the aggregation process. Some researchers have also investigated artificial oligomers prepared under nonphysiological conditions. In contrast to these "in solution" studies, we have focused on "membrane-mediated" amyloidogenesis. In an earlier study, other researchers identified a specific form of Aβ that was bound to monosialoganglioside GM1, a sugar lipid, in brains of patients who exhibited the early pathological changes associated with AD. This Account summarizes 15 years of our research on this topic. We have found that Aβ specifically binds to GM1 that occurs in clusters, but not when it is uniformly distributed. Clustering is facilitated by cholesterol. Upon binding, Aβ changes its conformation from a random coil to an α-helix-rich structure. A CH-π interaction between the aromatic side chains of Aβ and carbohydrate moieties appended to GM1 appears to be important for binding. In addition, as Aβ accumulates and reaches its first threshold concentration (Aβ/GM1 = ∼0.013), aggregated β-sheets of ∼15 molecules appear and coexist with the helical form. However, this β-structure is stable and does not form larger aggregates. When the disease progresses further and the Aβ/GM1 ratio exceeds ∼0.044, the β-structure converts to a second β-structure that can seed aggregates. The seed recruits monomers from the aqueous phase to form toxic amyloid fibrils that have larger surface hydrophobicity and can contain antiparallel β-sheets. In contrast, amyloid fibrils formed in aqueous solution are less toxic and have parallel β-sheets. The less polar environments of GM1 clusters play an important role in the formation of these toxic fibrils. Membranes that contain GM1 clusters not only accelerate the aggregation of Aβ by locally concentrating Aβ molecules but also generate amyloid fibrils with unique structures and significant cytotoxicity. The inhibition of this aggregation cascade could be a promising strategy for the development of AD-modulating therapies.

Curcumin and its derivatives: their application in neuropharmacology and neuroscience in the 21st century

Curcumin (diferuloylmethane), a polyphenol extracted from the plant Curcuma longa, is widely used in Southeast Asia, China and India in food preparation and for medicinal purposes. Since the second half of the last century, this traditional medicine has attracted the attention of scientists from multiple disciplines to elucidate its pharmacological properties. Of significant interest is curcumin's role to treat neurodegenerative diseases including Alzheimer's disease (AD), and Parkinson's disease (PD) and malignancy. These diseases all share an inflammatory basis, involving increased cellular reactive oxygen species (ROS) accumulation and oxidative damage to lipids, nucleic acids and proteins. The therapeutic benefits of curcumin for these neurodegenerative diseases appear multifactorial via regulation of transcription factors, cytokines and enzymes associated with (Nuclear factor kappa beta) NFκB activity. This review describes the historical use of curcumin in medicine, its chemistry, stability and biological activities, including curcumin's anti-cancer, anti-microbial, anti-oxidant, and anti-inflammatory properties. The review further discusses the pharmacology of curcumin and provides new perspectives on its therapeutic potential and limitations. Especially, the review focuses in detail on the effectiveness of curcumin and its mechanism of actions in treating neurodegenerative diseases such as Alzheimer's and Parkinson's diseases and brain malignancies.

Cholesterol accelerates the binding of Alzheimer's β-amyloid peptide to ganglioside GM1 through a universal hydrogen-bond-dependent sterol tuning of glycolipid conformation

Age-related alterations of membrane lipids in brain cell membranes together with high blood cholesterol are considered as major risk factors for Alzheimer's disease. Yet the molecular mechanisms by which these factors increase Alzheimer's risk are mostly unknown. In lipid raft domains of the plasma membrane, neurotoxic Alzheimer's beta-amyloid (Abeta) peptides interact with both cholesterol and ganglioside GM1. Recent data also suggested that cholesterol could stimulate the binding of Abeta to GM1 through conformational modulation of the ganglioside headgroup. Here we used a combination of physicochemical and molecular modeling approaches to decipher the mechanisms of cholesterol-assisted binding of Abeta to GM1. With the aim of decoupling the effect of cholesterol on GM1 from direct Abeta-cholesterol interactions, we designed a minimal peptide (Abeta5-16) containing the GM1-binding domain but lacking the amino acid residues involved in cholesterol recognition. Using the Langmuir technique, we showed that cholesterol (but not phosphatidylcholine or sphingomyelin) significantly accelerates the interaction of Abeta5-16 with GM1. Molecular dynamics simulations suggested that Abeta5-16 interacts with a cholesterol-stabilized dimer of GM1. The main structural effect of cholesterol is to establish a hydrogen-bond between its own OH group and the glycosidic-bond linking ceramide to the glycone part of GM1, thereby inducing a tilt in the glycolipid headgroup. This fine conformational tuning stabilizes the active conformation of the GM1 dimer whose headgroups, oriented in two opposite directions, form a chalice-shaped receptacle for Abeta. These data give new mechanistic insights into the stimulatory effect of cholesterol on Abeta/GM1 interactions. They also support the emerging concept that cholesterol is a universal modulator of protein-glycolipid interactions in the broader context of membrane recognition processes.

Syntheses and evaluation of novel isoliquiritigenin derivatives as potential dual inhibitors for amyloid-beta aggregation and 5-lipoxygenase

A series of new isoliquiritigenin (ISL) derivatives were synthesized and evaluated as dual inhibitors for amyloid-beta (Aβ) aggregation and 5-lipoxygenase (5-LO). It was found that all these synthetic compounds inhibited Aβ (1-42) aggregation effectively with their IC₅₀ values ranged from 2.2 ± 1.5 μM to 23.8 ± 2.0 μM. These derivatives also showed inhibitory activity to 5-LO with their IC50 values ranged from 6.1 ± 0.1 μM to 35.9 ± 0.3 μM. Their structure-activity relationships (SAR) and mechanisms of inhibitions were studied. This study provided potentially important information for further development of ISL derivatives as multifunctional agents for Alzheimer's disease (AD) treatment.

GM1 cluster mediates formation of toxic Aβ fibrils by providing hydrophobic environments

The conversion of soluble, nontoxic amyloid β-proteins (Aβ) to aggregated, toxic forms rich in β-sheets is considered to be a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid-protein interactions play a crucial role in the aggregation of amyloidogenic proteins like Aβ. Our group has previously reported that amyloid fibrils of Aβ formed on membranes containing clusters of GM1 ganglioside (M-fibrils) exhibit greater cytotoxicity than fibrils formed in aqueous solution (W-fibrils) [ Okada ( 2008 ) J. Mol. Biol. 382 , 1066 - 1074 ]. W-fibrils are considered to consist of in-register parallel β-sheets. However, the precise molecular structure of M-fibrils and force driving the formation of toxic fibrils remain unclear. In this study, we hypothesized that low-polarity environments provided by GM1 clusters drive the formation of toxic fibrils and compared the structure and cytotoxicity of W-fibrils, M-fibrils, and aggregates formed in a low-polarity solution mimicking membrane environments. First, we determined solvent conditions which mimic the polarity of raftlike membranes using Aβ-(1-40) labeled with the 7-diethylaminocoumarin-3-carbonyl dye. The polarity of a mixture of 80% 1,4-dioxane and 20% water (v/v) was found to be close to that of raftlike membranes. Aβ-(1-40) formed amyloid fibrils within several hours in 80% dioxane (D-fibrils) or in the presence of raftlike membranes, whereas a much longer incubation time was required for fibril formation in a conventional buffer. D-fibrils were morphologically similar to M-fibrils. Fourier-transform infrared spectroscopy suggested that M-fibrils and D-fibrils contained antiparallel β-sheets. These fibrils had greater surface hydrophobicity and exhibited significant toxicity against human neuroblastoma SH-SY5Y cells, whereas W-fibrils with less surface hydrophobicity were not cytotoxic. We concluded that ganglioside clusters mediate the formation of toxic amyloid fibrils of Aβ with an antiparallel β-sheet structure by providing less polar environments.

Using bacterial inclusion bodies to screen for amyloid aggregation inhibitors

Background

The amyloid-β peptide (Aβ42) is the main component of the inter-neuronal amyloid plaques characteristic of Alzheimer's disease (AD). The mechanism by which Aβ42 and other amyloid peptides assemble into insoluble neurotoxic deposits is still not completely understood and multiple factors have been reported to trigger their formation. In particular, the presence of endogenous metal ions has been linked to the pathogenesis of AD and other neurodegenerative disorders.

Results

Here we describe a rapid and high-throughput screening method to identify molecules able to modulate amyloid aggregation. The approach exploits the inclusion bodies (IBs) formed by Aβ42 when expressed in bacteria. We have shown previously that these aggregates retain amyloid structural and functional properties. In the present work, we demonstrate that their in vitro refolding is selectively sensitive to the presence of aggregation-promoting metal ions, allowing the detection of inhibitors of metal-promoted amyloid aggregation with potential therapeutic interest.

Conclusions

Because IBs can be produced at high levels and easily purified, the method overcomes one of the main limitations in screens to detect amyloid modulators: the use of expensive and usually highly insoluble synthetic peptides.

Transient focal membrane deformation induced by arginine-rich peptides leads to their direct penetration into cells

Endocytosis has been implicated in the cellular uptake of arginine-rich, cell-penetrating peptides (CPPs). However, accumulating evidence suggests that certain conditions allow the direct, non-endocytic penetration of arginine-rich peptides through the plasma membrane. We previously showed that Alexa Fluor 488-labeled dodeca-arginine (R12-Alexa488) directly enters cells at specific sites on the plasma membrane and subsequently diffuses throughout cells. In this study, we found that the peptide influx was accompanied by the formation of unique, "particle-like" multivesicular structures on the plasma membrane, together with topical inversion of the plasma membrane. Importantly, the conjugation of dodeca-arginine (R12) to Alexa Fluor 488 or a peptide tag derived from hemagglutinin (HAtag) significantly accelerated particle formation, suggesting that the chemical properties of the attached molecules (cargo molecules) may contribute to translocation of the R12 peptide. Coincubation with R12-HAtag allowed the membrane-impermeable R4-Alexa488 to permeate cells. These results suggest that R12 peptides attached to hydrophobic cargo molecules stimulate dynamic morphological alterations in the plasma membrane, and that these structural changes allow the peptides to permeate the plasma membrane. These findings may provide a novel mode of cell permeabilization by arginine-rich peptides as a means of drug delivery.

Quercetin inhibits amyloid fibrillation of bovine insulin and destabilizes preformed fibrils

Growing interest and research efforts have recently been focused on elucidating the molecular mechanism of amyloid formation and the screening of effective inhibitors to interrupt amyloid structures. In the present study, the anti-amyloidogenic effects of quercetin were investigated in vitro using bovine insulin as a model protein. The results demonstrated that quercetin dose-dependently inhibited amyloid formation of insulin. Moreover, quercetin destabilized the preformed insulin fibrils and transformed the fibrils into amorphous aggregates. Hemolysis was observed when human erythrocytes were co-incubated with insulin fibrils. Quercetin inhibited fibril-induced hemolysis in a dose-dependent manner. SDS-PAGE showed that insulin fibrils induced the aggregation of cytoskeletal proteins of erythrocyte membranes and that quercetin attenuated this fibril-induced cytoskeletal aggregation. The results of the present work suggest that quercetin may serve as a lead structure for the design of novel anti-amyloidogenic drugs.

Oxidative stress and cell membranes in the pathogenesis of Alzheimer's disease

Amyloid β proteins and oxidative stress are believed to have central roles in the development of Alzheimer's disease. Lipid membranes are among the most vulnerable cellular components to oxidative stress, and membranes in susceptible regions of the brain are compositionally distinct from those in other tissues. This review considers the evidence that membranes are either a source of neurotoxic lipid oxidation products or the target of pathogenic processes involving amyloid β proteins that cause permeability changes or ion channel formation. Progress toward a comprehensive theory of Alzheimer's disease pathogenesis is discussed in which lipid membranes assume both roles and promote the conversion of monomeric amyloid β proteins into fibrils, the pathognomonic histopathological lesion of the disease.

Linking amyloid protein aggregation and yeast survival

Protein aggregation and amyloid formation lie behind an increasing number of human diseases. Here we describe the application of an "aggregation reporter", in which the test protein is fused to dihydrofolate reductase, as a general method to assess the intracellular solubility of amyloid proteins in eukaryotic background. Because the aggregation state of the target protein is linked directly to yeast cells survival in the presence of methotrexate, protein solubility can be monitored in vivo without the requirement of a functional assay for the protein of interest. In addition, the approach allows the in vivo visualization of the cellular location and aggregated state of the target protein. To demonstrate the applicability of the assay in the screening of genes or compounds that modulate amyloid protein aggregation in living cells, we have used as models the Alzheimer's amyloid β peptide, polyglutamine expansions of huntingtin, α-synuclein and non-aggregating variants thereof. Moreover, the anti-aggregational properties of small molecules and the effects of the yeast protein quality control machinery have also been evaluated using this method.

Formation of Toxic Amyloid Fibrils by Amyloid β-Protein on Ganglioside Clusters

It is widely accepted that the conversion of the soluble, nontoxic amyloid β-protein (Aβ) monomer to aggregated toxic Aβ rich in β-sheet structures is central to the development of Alzheimer's disease. However, the mechanism of the abnormal aggregation of Aβ in vivo is not well understood. Accumulating evidence suggests that lipid rafts (microdomains) in membranes mainly composed of sphingolipids (gangliosides and sphingomyelin) and cholesterol play a pivotal role in this process. This paper summarizes the molecular mechanisms by which Aβ aggregates on membranes containing ganglioside clusters, forming amyloid fibrils. Notably, the toxicity and physicochemical properties of the fibrils are different from those of Aβ amyloids formed in solution. Furthermore, differences between Aβ-(1–40) and Aβ-(1–42) in membrane interaction and amyloidogenesis are also emphasized.

Amyloidogenic protein-membrane interactions: mechanistic insight from model systems

The toxicity of amyloid-forming proteins is correlated with their interactions with cell membranes. Binding events between amyloidogenic proteins and membranes result in mutually disruptive structural perturbations, which are associated with toxicity. Membrane surfaces promote the conversion of amyloid-forming proteins into toxic aggregates, and amyloidogenic proteins, in turn, compromise the structural integrity of the cell membrane. Recent studies with artificial model membranes have highlighted the striking resemblance of the mechanisms of membrane permeabilization of amyloid-forming proteins to those of pore-forming toxins and antimicrobial peptides.

The pathological roles of ganglioside metabolism in Alzheimer's disease: effects of gangliosides on neurogenesis

Conversion of the soluble, nontoxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is a key step in the onset of Alzheimer's disease (AD). It has been suggested that Aβ induces changes in neuronal membrane fluidity as a result of its interactions with membrane components such as cholesterol, phospholipids, and gangliosides. Gangliosides are known to bind Aβ. A complex of GM1 and Aβ, termed “GAβ”, has been identified in AD brains. Abnormal ganglioside metabolism also may occur in AD brains. We have reported an increase of Chol-1α antigens, GQ1bα and GT1aα, in the brain of transgenic mouse AD model. GQ1bα and GT1aα exhibit high affinities to Aβs. The presence of Chol-1α gangliosides represents evidence for genesis of cholinergic neurons in AD brains. We evaluated the effects of GM1 and Aβ1–40 on mouse neuroepithelial cells. Treatment of these cells simultaneously with GM1 and Aβ1–40 caused a significant reduction of cell number, suggesting that Aβ1–40 and GM1 cooperatively exert a cytotoxic effect on neuroepithelial cells. An understanding of the mechanism on the interaction of GM1 and Aβs in AD may contribute to the development of new neuroregenerative therapies for this disorder.

Ganglioside-mediated aggregation of amyloid β-proteins (Aβ): comparison between Aβ-(1-42) and Aβ-(1-40)

Conversion of the soluble, non-toxic amyloid β-protein (Aβ) into an aggregated, toxic form rich in β-sheets is considered a key step in the development of Alzheimer's disease. Accumulating evidence suggests that lipid rafts in membranes play a pivotal role in this process. We have proposed that Aβ-(1-40) specifically bound to a ganglioside cluster forms cytotoxic fibrils via a conformational transition from an α-helix-rich structure to a β-sheet-rich one. In the present study, we compared the interaction of Aβ-(1-40) and Aβ-(1-42) with both model and living cell membranes. Aβ-(1-42) exhibited lipid specificity and affinity similar to Aβ-(1-40), though its amyloidogenic activity was more than 10-fold that of Aβ-(1-40). Antibody staining experiments, using the A11 antibody specific to Aβ oligomers, demonstrated that oligomers were not detected during the aggregation process, and cell death was observed only after significant accumulation of the proteins, suggesting that the fibril-induced disruption of cell membranes leads to the cytotoxicity. Furthermore, we succeeded in visualizing fibrils formed on cell membranes using total internal reflection fluorescence microscopy. Aβ-(1-40) formed long fibrils extruding to the aqueous phase, whereas Aβ-(1-42) fibrils appeared to be laterally co-assembled and short.

Macroautophagy is not directly involved in the metabolism of amyloid precursor protein

Alterations in the metabolism of amyloid precursor protein (APP) are believed to play a central role in Alzheimer disease pathogenesis. Burgeoning data indicate that APP is proteolytically processed in endosomal-autophagic-lysosomal compartments. In this study, we used both in vivo and in vitro paradigms to determine whether alterations in macroautophagy affect APP metabolism. Three mouse models of glycosphingolipid storage diseases, namely Niemann-Pick type C1, GM1 gangliosidosis, and Sandhoff disease, had mTOR-independent increases in the autophagic vacuole (AV)-associated protein, LC3-II, indicative of impaired lysosomal flux. APP C-terminal fragments (APP-CTFs) were also increased in brains of the three mouse models; however, discrepancies between LC3-II and APP-CTFs were seen between primary (GM1 gangliosidosis and Sandhoff disease) and secondary (Niemann-Pick type C1) lysosomal storage models. APP-CTFs were proportionately higher than LC3-II in cerebellar regions of GM1 gangliosidosis and Sandhoff disease, although LC3-II increased before APP-CTFs in brains of NPC1 mice. Endogenous murine Aβ40 from RIPA-soluble extracts was increased in brains of all three mice. The in vivo relationship between AV and APP-CTF accumulation was also seen in cultured neurons treated with agents that impair primary (chloroquine and leupeptin + pepstatin) and secondary (U18666A and vinblastine) lysosomal flux. However, Aβ secretion was unaffected by agents that induced autophagy (rapamycin) or impaired AV clearance, and LC3-II-positive AVs predominantly co-localized with degradative LAMP-1-positive lysosomes. These data suggest that neuronal macroautophagy does not directly regulate APP metabolism but highlights the important anti-amyloidogenic role of lysosomal proteolysis in post-secretase APP-CTF catabolism.

Exosome secretion ameliorates lysosomal storage of cholesterol in Niemann-Pick type C disease

Niemann-Pick type C1 disease is an autosomal-recessive lysosomal storage disorder. Loss of function of the npc1 gene leads to abnormal accumulation of free cholesterol and sphingolipids within the late endosomal and lysosomal compartments resulting in progressive neurodegeneration and dysmyelination. Here, we show that oligodendroglial cells secrete cholesterol by exosomes when challenged with cholesterol or U18666A, which induces late endosomal cholesterol accumulation. Up-regulation of exosomal cholesterol release was also observed after siRNA-mediated knockdown of NPC1 and in fibroblasts derived from NPC1 patients and could be reversed by expression of wild-type NPC1. We provide evidence that exosomal cholesterol secretion depends on the presence of flotillin. Our findings indicate that exosomal release of cholesterol may serve as a cellular mechanism to partially bypass the traffic block that results in the toxic lysosomal cholesterol accumulation in Niemann-Pick type C1 disease. Furthermore, we suggest that secretion of cholesterol by exosomes contributes to maintain cellular cholesterol homeostasis.

Catechol derivatives inhibit the fibril formation of amyloid-beta peptides

The inhibition of fibril formation of amyloid beta proteins (A beta) would be attractive therapeutic targets for the treatment of Alzheimer's disease (AD). Dopamine (DA) and other catechol derivatives were used as inhibitory factors for A beta fibril formation. The fibril formation of A beta was monitored by Thioflavin T fluorescence, a transmission electron microscopy (TEM) and a total internal reflection fluorescence microscopy (TIRFM). Catechol and its derivatives showed the dose-dependent inhibitory effects on the spontaneous A beta fibril formation. The inhibitory activity depended on the chemical structure of catechol derivatives both in the presence and absence of the liposome a model of biomembrane. Formation of catechol quinone-conjugated-A beta adduct by a Schiff-base is a key step for the inhibition effect of A beta fibril formation.

Inhibitors of protein aggregation and toxicity

The aggregation of numerous peptides or proteins has been linked to the onset of disease, including Abeta (amyloid beta-peptide) in AD (Alzheimer's disease), asyn (alpha-synuclein) in Parkinson's disease and amylin in Type 2 diabetes. Diverse amyloidogenic proteins can often be cut down to an SRE (self-recognition element) of as few as five residues that retains the ability to aggregate. SREs can be used as a starting point for aggregation inhibitors. In particular, N-methylated SREs can bind to a target on one side, but have hydrogen-bonding blocked on their methylated face, interfering with further assembly. We applied this strategy to develop Abeta toxicity inhibitors. Our compounds, and a range of compounds from the literature, were compared under the same conditions, using biophysical and toxicity assays. Two N-methylated D-peptide inhibitors with unnatural side chains were the most effective and can reverse Abeta-induced inhibition of LTP (long-term potentiation) at concentrations as low as 10 nM. An SRE in asyn (VAQKTV) was identified using solid-state NMR. When VAQKTV was N-methylated, it was able to disrupt asyn aggregation. N-methylated derivatives of the SRE of amylin are also able to inhibit amylin aggregation.